Exposure to asbestos is the primary cause of malignant mesotheliomas (MMs), highly invasive tumors that arise from the mesothelial cell lining of the pleural, peritoneal, and pericardial cavities. MMs exhibit frequent mutation of the NF2 tumor suppressor gene (TSG) and homozygous deletion of the INK4a/ARF locus, which encodes the TSG products p16INKa and p14ARF. The tumorigenic potential of p16INK4a alterations has been previously demonstrated in MM cells, whereas evidence in support of a pathogenetic role for p14ARF loss in MM is less well established. Another putative TSG, GPC3, is markedly down regulated in many MMs. The hypothesis is that alterations of NF2, p14ARF and GPC3 are manifestations of key disturbances in cellular physiology that collectively contributes to the development and/or progression of MM. Characterization of the involvement of these TSGs in MM will contribute to a better understanding of molecular mechanisms underlying the pathogenesis of this malignancy, which is the broad, long-term objective of this project. The specific aims are: 1) Characterize mechanisms by which NF2 inactivation contributes to the pathogenesis of MM. Evaluate the effect of merlin expression on the invasive/metastatic potential of MM cells in vivo and on substrate adherence, membrane ruffling, motility and invasiveness in vitro. To discern oncogenic mechanisms associated with NF2 inactivation, determine how merlin function is modulated by phosphorylation as an effector of Rac/Cdc42/Pak1 signaling and whether merlin expression/phosphorylation alters downstream signaling involving JNK and AP-1. 2) Determine the involvement of GPC3 down regulation in the malignant phenotype of MM cells and delineate the potential role of GPC3 in the transduction of insulin-like growth factor 1 receptor (IGF1R)-mediated signaling. Assess the effect of GPC3 on MM cell growth/survival, adhesion, and invasiveness. Elucidate mechanisms by which GPC3 may contribute to oncogenesis through participation in IGF signaling. 3) Ascertain if ARF (+/-) and Nf2 (+/-) knockout mice are predisposed to the induction of MM by asbestos. Asbestos-treated ARF (+/-) and NJ2 (+/-) mice will be evaluated with regard to the incidence and invasive/metastatic potential of MMs observed in these two mouse models and the requirement for biallelic inactivation of the predisposing TSG and/or cooperation of oncogenes or other TSGs. Overall, these studies are expected to provide insights regarding mechanisms by which specific TSGs contribute to the pathogenesis of MM and delineate potential avenues for therapeutic intervention.